Door systems to control the lowering of doors in emergency conditions have been used for many years. Among the doors so controlled are fire doors including fire doors of the type comprising a plurality of horizontal slats pivotally connected to one another to enable the fire door to be reeled in when raised and unreeled when lowered. There are numerous prior art mechanisms known and used for raising such fire doors to the normal or non-emergency conditions and for lowering the doors during a fire. In such operating mechanisms, an electric motor is commonly used to raise the door to an open locked position. When a fire occurs, these operating mechanisms disengage the motor from the fire door and allow the door to close either under the urging of an auxiliary spring activated by mechanical means or by gravitational surpassing the force exerted on the door by a torsional spring counter balancing mechanism.
Conventional rolling steel fire doors are usually utilized in pairs so that one fire door closes an opening in a wall on one side of the wall and the other fire door closes the same opening on the other side of the wall. Most importantly, it is usually required under local fire codes that the two retractable fire doors close simultaneously. To simultaneously close the two fire doors, previously known fire door systems connect the pair of doors with a sash chain which passes through an aperture in the wall. The sash chain has one or more links comprised of two pieces of metal held together by low melting-point solder. When the solder in any one of the fusible links melts, such as when the ambient temperature reaches or surpasses a predetermined value, the links separate or break, thereby releasing the tension on the sash chain on both sides of the wall opening. With this tension removed, a compression spring on each door assembly releases a plunger which in turn disengages a brake which keeps the rolling fire doors locked in their open positions. Accordingly, both steel fire doors simultaneously close, due to the force of gravity. However, as is evident, a typical problem with this through-the-wall sash chain connection method is that the links or the sash chain can become ensnared in the aperture through the wall. Additionally, if the chains are mistakenly painted or become entangled in various proximate structures, either or both of the doors may not properly close in the event of a fire.
An additional drawback of such fire door systems is evident when more than two doors at more than one opening are to operate together. For example, when a building is provided with more than one opening, and if all the pairs of doors at the various openings are to drop simultaneously on the breaking of a single fusible link, a chain must be run through the wall from one door to the next. When the fire doors of more than two openings are to operate jointly, it can be readily seen that the chain configuration can easily become complex, cumbersome and impractical and, thereby be subject to interference and malfunction, with the end result being that the fire doors do not properly close when required.